Biasing potential switching system



Feb. 22, '1949. G. EAKlN Er AL.

BIASING POTENTIAL SWITCHING SYSTEM 6 Sheets-Sheet 1 Filed Jan. 5, 1946 N. m Tw. l NB h @PIM Dm A m H 5w?! M MH V F so@ a mm m mv @www mm BON mm2 o .o Ga Ymx H Y B I Illllll v .I llt! 1 Luv ,aE :L im t mhz. W @QOWN S FID Dl Nv: )www1 im. MQ= mgl DIH. D mm Nlm aoco om w aoco om E Sooo@ BooQm. lwl L MSE, 2.2, .IEIL Dm :22125 Sm :zu @im N555 ww m5, @um Nv l l 1 l ATTORNEY.

Feb. 22, 1949. G. DEAKIN Er Al.

BISING `POTENIIAL SWITCHING SYSTEM ,6 Sheets-Sheet 2 Filed Jan. 5. 1946 ATTORNEYv Feb. 22, 1949u G. DEAKIN E-r AI. 2,462,074

BIASING POTNTIAL SWITGHING SYSTEM Filed Jan. 5, 194e 6 Sheets-Sheet 3 FIG. 4.

ASSIGNMENT DF TEsT PoTENTIALs PoR REGISTER cIRcuIT PN vous gg UNE FINDER SITJQS sE'L-Ecggegs) sIE'I-.EfII-'uLNITsI No. T CPN) DLPsI T (PN) D (Ps) TLPN) GLPS) T CPN) B I=sl 4 20 Pci ML-K DIGIT Y DIGIT Y e I Pez NIL-D DIGIT I DIeITI mm I 8 2 PL-K DIGIT 2 DIGIT 2 DIGIT 2 -IO 3 DIeI'I= 3 DIGIT a DIGIT s -I2 4 DIGIT 4 DIeI14 wenn I4 5 DIGIT 5 DIGIT 5 DIGIT 5 AISSK -IG 6 DIGIT e DIGITG DIeITG pL-/g -La I DIGIT 7 DIeIT T DIeITI PL-Tg -20 8 DIGIT 5 DIGIT DIGIT a -22 s DIGIT s DIeITs DIeITD -24 I0 PI.A DIGIT o DIGIT D DIeITD -zG II PL- D vDIGIT x DIGIT x DIeITs 3f, -za I2 PL-e MM- z oT-u mens 3g NIL- FREE -eo Is PLI NIM-3 oT-s mene-f8 'gt-DUST -32 I4 I1 I-1 DIGIT 54 I5 Lz I 2 I:IIeITs5/6 -36 IG T-a 1-5 NE No NE No -36 IT 1-4 I4 -AQ I6 I-5 I-5 -42 ID r-e I-G Pc- CALLING PoTENTIAL PL A,D.G on J STATION IDENTIFICATION ML- MAIN LINE on Pax I CALLING LINE NUMBER IDENTIFICATION PL 4 PARTY LINE oT- ouTTRuNK To UNIVERSAL K KEY sET oeouTrRuNK To sTIzoweEIa D DIAL MM2-MULTIPLE METERINe TwIcE M IP METERINGT I ya? IcTED sERvcREn CE GEDENDTKI N PE2-Raymann sERvIcE HENRY -IHERBIG NE-NoN-ExIsTINe BY j I A TTORNE Y.

Febb 229 19149. Q DEAKlN ET AL 2,462,74

BIASING POTENTIAL SWITCHNG SYSTEM Filed Jan. 5, 1946 GSheets-Sheet 4 Fue.. 5.

LUE oF E\ lN vous INVENTOR. GERALD .DEAK|N.

BY- HENRY F. HERBIG.

A T To ENEK Feb. 22, 1949. 3.- DEAK1N ET AL 2,462,074

BIASING POTENTIAL SWITCHING SYSTEM Filed Jan. 5. 1946 6 Sheets-Sheet 5 OF El IN VO Fle.

INVENTOR. GERALD DEAmN BY HENRY FHERBIG A Trae/EK Feb. 22, 1949. G. DEAKIN ET AL. 2,462,074

BIASING POTENTIAL SWITCHING SYSTEM Filed Jan. 5. 1946 6 Sheets-Sheet 6 FIG. S.

El VALUE OF E \N VOLTS El-SRBO FIG. l0.

IN V EN TOR.

GERALD vDKUN By HENRY r- HERB!G A T To ENA-X Patented Feb. 22, 1949 UNITED STATES PATENT OFFICE BIASING POTENTIAL SWITCHING SYSTEM Application January 5, 1946, Serial No. 639,287

7 Claims. 1

a series of discrete potentials on both register `r and selector sides.

Another object of this invention is a biasing system permitting the establishment of an accurate output potential which can be used for switching, indicating or any other operative purpose and which depends upon the application of a predetermined potential dilerence to the control circuit.

More specifically the biasing system includes a circuit or circuits superimposing a relatively small but constant direct current potential over one or both series of discrete potentials.

Another object of this invention is a means for superimposing a continuous potential over at least one of the series of discrete potentials to maintain the output potential below its operative value in case the discrete potentials are absent.

A more specic object of this invention is to bias the several electron discharge paths of a twin multi-electrode tube from a common circuit.

`Still another object of this invention is the timing of the control operation, and more spen ciiically, a delay circuit preventing false operation in gas discharge tubes operated from the control circuit.:

Afurther object is `a switching means operated from the control circuit at appropriate moments to disconnect certain sources of potential from the system so as to reduce wear on the battery and on any other equipment.

These and other objects of the invention will more clearly appear from the claims, the following description and the appended drawings, in which: e

Fig. 1 diagrammatically illustrates as much of a control circuit embodying the various features of the present invention as is necessary for a clear understanding thereof 2 Fig. 2 is a graph showing the control electrode voltage of a gas discharge tube plotted against the grid voltage of a triode;

Fig. 3 is a circuit used in the determination of this graph;

Fig. 4 shows an example of an assignment of test potentials for a large network office; and

Figs. 5 to l0 are test sheets showing the breakdown potential of a gas tube as a result of the matching of test potentials belonging to two series of discrete potentials.

In an automatic telephone system, calling and called subscribers stations I and 2 (Fig. 1) may be inter-connected by means including a plurality of selector switches 3 and 4. The numerical operations of the selectors are controlled by the calling subscribers customary impulse sender or other number indicating device through the agency of a register provided at the central exchange. The calling station I is connected to the brushes of selector 3 by non-numerical switches and a connecting circuit 6, and the register is attached to the connecting circuit 6 through switching means 5.

The selector switches are of the customary type having a bank of terminals with which a set of brushes cooperates. In the drawings, three brushes are shown for selector 3 although the selector may have a larger number of brushes. The selector brushes are moved over the terminals while a magnet P is energized and are stopped on a set of terminals when this magnet becomes rie-energized.

The register may comprise a customary arrangement of switches, relays or other electrical digit-storing devices successively operable by the calling device in accordance with the successive digits of the called number. A brush and a set of terminals of one of these register switches is shown in Fig. l. This is the register switch -which responds to the digit destined for selector 3. Register brush RB is advanced toward one of a series of say ten terminals (only four of Y 3 moving over the terminals and will continue until GVR becomes energized.

The energization of relay' GVR is controlled by a gas valve GV having a cathode C, an anode A and a control electrode CE. The control electrode or grid CE is connected over a resistance RI, of say 300,000fohms; tog'theanodesofi twin triode VI,` V2.` High tension battery HTB feeds over ARI front 2 and resistance R3, of say 100,000 ohms, the anodes of twin triode VI, V2*l and over a resistance R5, of Isay y1,000,0hms to.. the anode A of gas valve GV. Cathode C'is,

grounded over a relatively highfresistancerelay AGVR, of about 1,000 ohms. In-thislway, a potential developed in the outputy circuitireceivinglthe electrons of the two electron dischargel'paths of the twin triode is used to control thedischarge of tube GV; relay GVR will be energized when the gas valve ionizes. As a result of the dis-4 charge of tube GV, the relatively low resistance relay and `the .selectory magnet P. will be-.ade-

energised.- The resistance: ,ofy AR-...may be f-abouta: 20 ohms and that of selector-magnet?about 130 china;

factors upon: the characteristics of 'twin-. tricde VI g V2 andgas ,valve GV,y andinay' be :varied .Withjin g wide limits without Y exceeding the` scope of this( invention.V

Relay ARI, via back rcontacts I, 2, disconnects.

medium tension 1battery.-MTB andhigh tension gizingcircuit for AR-andrP.-A When relay GVRis The circuits operating, the two grids on the marked. :(or. register). side. andthe .test brush'. .(or.

selector) side are symmetrical. Brushl RBfwhen,

mo ving..` over its associated terminals applies to grid-,GVI and.. cathode .CVZ a series of-.discr,ete.-. potentials. Similarly, .another seriespf discrete` potentials is applied frombrush SB togrid GV2 L. and cathode. CVI. Thus the grid iny one electron.4

path and the ,cathode in, the ,otherelectron vpath of. thetwin multi-electrode tube ,have .a .predeter-l mined constant potential relationship.`A In the embodiment shown, the gridof .VI (orVZ) and.,

thev `milione of .V2 (crvl i have, substantiauythe samenegative potentahnamelyfthe discrete por tential .Y applied by" selector.v and Aregister,brushes resine@tively-4 The discrete potentials .g are. appliedtoA CVL.;

CVZdirec-tlyand Y.to GVI, GV2 over gridare-sist-v an'esRZ; R21, cf say 50,000 ohms4 Inaccldance v.with .this invention'. a bias Girgetionship and preferably vare substantially'l-equal; Each; of; the vregistering and selecting grids GV i.' GV2 is vrbia-sed 1in exactly the same -qmanner Abysuperimposinga constant directl current=potential over.'y each;Y of fthe discrete'ipotentials applied.'y

tovthe grids. In thefembodiment shown the bias-.s

ing/potentialsapplied to grids GVI, GV2 aresub-v stantiallythesame and have the relatively small .value of approxiiriately v-\1.'8 volts.-V Howeventhe biasing potentials may differ, both absolutely fandrelatively.- depending. upon the .ltubeA :and v4,circuit 4 structure, without exceeding the scope of this invention. The source of bias potential may be any appropriate direct current source, a battery, a dry cell or an A. C. fed rectifier of any desired construction.

In the embodiment shown in Fig. 1, the biasing potentialis applied fromrectie1zpairs SI, S2 andSI; S2"to the grids -fover balancingfresistances RI, Rl of any appropriate value, say 3,000 ohms inserted between brush RB, SB and grid resistance. R2, R25, respectively.

Balancing resistances R'I, R1 are shunted by a suitable smoothing-1 condenser C2, C2 of say 30-40 microfarad capacity which serves to eliminate or reduce-'any ripple passing the rectifier pairs-SI, S2 tand SI', S2'. The latter are supplied from the cathode heater circuit over transformers TTI, TT2.

Onefend of R'I is connected over rectiers SI', S2 to the ends of the secondary of transformer TTZ: The `cen-ter tap Toi .-,thesecondaryis connected'to the p other gend; oiV resistance R'Isr Similarly, v .balancingaresistanceiy R'I 'i isfshuntedf.; byanother. capacitor C21 similarto; ibOthR t and' 02T. 'arefconnected' with.ione;;errd;.over-;rect ners Si', S2' to the ends;.;of,.:zthe:.secondaryifor; transformer TTI;l Thefother Vends. ofzresistance R1" and. capacitor C21 yleadftotheucenter tapfroff' that secondary.-

The Vtwo primaries of i, transf ormer-s 5TT2, vTT I- are connected in parallel preferably over aysmali'i'i rheostat RH. ofosay 18y ohms; tofxthe'lcommon cathode: heating 'circuit jfor the 'twinrtriodes :V I, V21 and, more 'particularly'. tothe secondary-foin'. say about 6.2 -volts offcathode heater.,transformer.: TT3

The invention is .noti limitedt tozfthe resistance and.v capacityl valuesfspecified .and to. .the `.circuits shown.. In fact .it .isya characteristic.:*advantagef; of :thisV invention that 4v-these values and i: circuits may be changed.' `within f' wide -limits vand adapted toalltypesfof installations and applicatonsawithout aiecting the principle of the inventionarniz.l the refficiency fof- .its operation. f.

Balancingresistances RLz RT.; smoothing: condensers 02,: C2,.rectier lpairs SI-SZ, SI and@ SZ, and ,transformers TTZ, TTI;Ourthe-register ing as well as theselectingside, may-be attached to each. otherso asftof'form a .detachableunitz BUI, BU2. f

The biasing unitorzunits `arenot grounded.;A

If desired,v and, .especially when-twin multi-H- electrode structure, such as the twin triode .VI,'. V2 shown iniFig.. 1,v are used,- the-biasing units controlling 'the different input electrodes or 'grids may` alsor bev combined inta-.single Adetachable. structure. Rectifiers and transformers maybe mounted on a radio-type. plug-in sock-etof Athe kind disclosed'fin: the copending-wapplicationf.ofi

Henry HerbigffserialgNo. 628,923,led NoveznbergfA As shownin Fig. 1 the common receivinggcir'facuit rior-.'theelectrons fcontroliedabygthe interac-l tionof;v twoinput electrodes "produces:Y an; operate` ing `liotentialf which is appliedtoi the Acontrol-point of "a gas vdischarge xpath,-v the receiving :circuit` of which maybe Ausecito control aswitching operan` tion, suchv as the movementqof selector brush SB.v

A timing circuit `vtzonsistingofv resi-stancerR and'. condenser CI in shunt-.tothe Ygas Adi-schargeipath serves to assure switching at exactly the moment-.t the operatingl potential `is 7produced. This pre'- vents `false operation, such as premature" rin'g due to transient Voltage, :while selector"brush^SB1l in the course of the movement 'applies :a #series-goti?.r

discrete potentials to its associated input electrode, grid GV2. Resistance R6 is rather high, say 4 megohms, and condenser CI has a relatively low capacity of, say .001 microfarad. The elements of the timing circuit may have any appropriate values and connections depending on the characteristic of gas tube GV and other facto-rs controlling its operation. In the claims herein R6 is designated as a bleeder resistance.

As selector brush SB moves over its associated terminals, it will eventually arrive at a position where potential E2 on the register brush RB is the same as potential E3 on lthe selector brush SB; in this case the grids GI, G2 of VI and V2 are both negative by approximately 1.8 volts with respect to their cathodes CI, C2. Under this condition the plate current through resistor R3 is reduced to such an extent as to allow the potential EI at the control grid CE of the gas tube GV to rise to breakdown voltage. Thereupon, GV fires and operates relay GVR. The breakdown voltage of the gas tube is between 66 volts and 80 volts. An examination of the test sheets shown in Figs. 5-9 shows that when potentials E2 and E3 between limits of -4 volts and -42 volts are matched, the resulting breakdown potentia] EI of the gas tube reaches a minimum of v measuring circuit shown in Fig. 3. In this circuit, the grid of the VI section of a 6SL7-GT twin triode is connected over a portion of a :small balancing resistor RII and a grid resistor RI2 of about 50,000 ohms, to the cathode of VI and, simultaneously, to the minus pole of high tension battery HTB. The anode of VI' is connected over a resistor RI 2, of say 300.000 ohms, to control grid CEI of gas valve GVI. Cathode Cl and anode AI of GVI are connected in a manner not shown but similar to that illustrated in Fig. 1. Resistor RM', of say 100,000 ohms, connects the anode of VI with the positive pole of HTB. A 4.5 volt low tension battery bridges the balancing resistor RI I and supplies the biasing potential.

Resistance and battery values and their respective connection are, of course, open to wide variations depending upon the characteristic of the type of tube used, the functions to be measured and other factors controlling the measuring circuit.

The gas tube GV does not flre when the difference in potential between register and selector brushes RB and SB, respectively, in Fig. 1 E2 and E3, is equal to or greater than the difference between adjacent test potential-s; i. e. 1.7 volts (Figs. 9 and 10), 2 volts (Figs. 5 and 6), and 2.4 volts (Figs. 7 and 8).

As previously stated, the gas tube will not re when the potential of the control electrode is maintained below 66 volts. It has been found that the highest voltage El reached is 57 volts when the test potential separation is 1.7 volts with the register test potentials E2 and E3 at the lower end of the scale (Fig. 10).

. The calling subscriber controls potential E2 through the register and when this potential is These latter voltages are the limits set, ARI or its equivalent operates. The circuit for high tension battery HTB is closed, also the clutch circuit of the selector. The selector brush SB commences to move over the test terminals.

It is assumed for the purpose of explanation that the potential E2 placed on RB is -22 volts. When SB tests -2 volts, potential EI becomes -5 volts (Fig. 5). When SB tests -4 volts, EI becomes *3 volts. When SB reaches -20 volts, potential EI is increased to +32 volts but when it reaches -22 volts, potential EI jumps to +107 volts and the gas tube res and operates high speed relay GVR which opens the clutch circuit of the selector, cau-sing it to come to rest. All selections are made in a similar manner.

The potentials actually found in the selector bank need not be in numerical order. They may be mixed up in any manner since it is only when potentials E2 and E3 are matched that the gas tube can nre. The selector brush SB may rub with safety over open test terminals, -44 volt test terminals, grounded test terminals, or those connected to +70 volts for metering.

The purpose of resistor R4 and I4-volt battery MTB is to maintain potential EI below ring point when selector brush SB is disconnected from a bank terminal or, in other words, from a potential E3. Potential El is then known as the SBO (selector brush open) potential. The highest SBO potential reached under this condition is 49 volts (Fig. 10).

Resistor R4A connected to -44 volts is added so as to maintain the potential of EI below flring point when both SB and RB are opened. EI is then known as the SRBO (selector register brush open) potential and the highest potential reached under this condition is 31 volts (Fig. 9).

In effect brush SB when not engaging one of its contacts is grounded over R4. However, due to the relatively high resistance of R4 with respect to the control resistance of any source for marking a selector contact, the conta-cts are not short circuited when SB does engage them. For this reason R4 and similarly RllA may be designated as current limiting resi-stance. When RB is not engaging one of its contacts it is connected to -44 v. over RdA. This voltage is selected as a value which cannot be matched by the potential of any of the selector contacts so that the circuit is prevented from operating unless RB is in engagement with one of its contacts. For this rea-son the -44 v. may be designated as a mismatched potential.

The test potentials E2 and E3 may be assigned Ain many ways and for many purposes. A typical assignment for a large oflice in a large network given in Fig. 4. The assignment may be different for line finders, group selectors and final selectors and for the B and '1F-terminals in case of duplex testing. Referring to the column Group selectors, it will be seen that for the T-terminals potentials -6 to -24 are assigned to two extra digits controlled by the register; potentials -28 and -30 are assigned for double and triple metering and potentials -32 to -42 for six identication circuits, all of which may be used simultaneously without confusion.

The filament current supply is a Gil-cycle A. C. at 6.2 volts |5% but may be any type of alterhating current system of any desired frequency. Provision should be made for an emergency supply of prime power in case of outside power failures. The reserve equipment may be that provided for the bay motors and high tension battemi-supply ando-can.. be driven..fromfthe main exchaneebattery.

f f A l;comme rcialgtwinil triode, :such as RCAl type @SUL-GT, may be usedl to produce the electron discharge paths. Itha-s lbeen found possibleA to interchange RCA twin ftriodes, without making any adjustment whatsoever in resistances or potentials;

Other twin or single multi-electrodestructures maybe used `for the purpose of this invention without exceeding the scope thereof.

Afcommercial three-electrode gas valve, such as-RCA 1G21, may be used to provide the gas discharge path. The average ring potential of the control electrodeis 73 volts with a minimum efffvolts and a-maximumfof 80 volts. The gas valve should be shielded to'ground to prevent accidentalhring from external causes;

The invention isyofcourse, not limited to the use of the gas valve shown, nor to the use of a gas triode. Any appropriatetype of gas or electron discharge device may be operated from the output potential ofthe control circuit without departing from the spirit of' this invention.

The new systemfoperates successfully withy the individual battery cell potential varying from 1.7 toitheffully charged fpotential of the cell. The cellsof the-normal negative main exchange bat tery may beused fortest potential purposes providingthe number of potentials required do not cause the uppermost potential to foul the eX- change bus bar potential fof `-48, minimum 44, maximum 55 volts, when the individual cells are at their maximum potential.

For example, `if thecrnaximumv individual cell voltage can, bezkeptfat 2.5 or less, iifteen test potentials are available, but if twenty potentials are .required,-a small separate battery must be provided -fortest potential purposes. This battery maybe of the smallest commercial type since the current drain is very slight and it may be floated off a rectier charger. For emergency purposes, a twenty-one pole switch should be' provided -to transfer the test potential leads to the main exchange battery, the individual cell potentiall of which during'v the emergency must be maintained at approximately 2 volts.

The potentialoi the high tension D. C. supply for the'. plate -andtgas tubev circuits should be maintained between 133 and 146 volts. A rectiiier-is recommended. A high tension storage battery may be used.

It a characteristic feature of this invention that the steps .between discrete potential-s as well as `thebiasing potentials vare small; the potentials may be derived, as, shown in the embodiment of the` invention illustrated in the drawings, from the individual cells of a customary exchange bat tery. It is, however, not necessary to limit the invention tothespecied potentials ,and tothe sources of potential shown. Any appropriate type or 'current source and` potentials of any suitable magnitude may be usedy for ,the purposes of this invention, Without departing from its scope.A

Relay GVR operatedA by the gas tube should break its `back contact in not less than about 1.5 milliseconds. No particularV form of relay isspecied but it should be capable of breaking the circuit indicated in Fig. 1, which contains low resistance relay AR, of say ohms, in series with aclntchfmagnet P, ofisa'y f13o ohms;-both relay bcdiment 1 shown,-ifresistancerR8;har a valuefaof about 400 ohms and thecondenser- CS a capacity ofaboutl microfarad. `The inventionrhowever, is not limited to the specified yvalues or range of values. Any resistance and `capacity serving the purpose may be usefully applied. A back Contact pressure of 20 grams4 is recommended, but successful results have been obtained with a relay having a lesser-backY Vcontact pressure,;of -say l0 grams.

Cl and Cl' aremica capacitors, C2 and C2 are electrically 'polarized capacitors; but'may consist of paper Idepending upon the frequency of the A'. C. source feeding the rectiers.V C3 is a paper capacitor. The seven: independent resistances Rie-Rl", Ri-Rl are of the radiov type.

There is Vample margin in the resistances shown in Fig.Y l `so that;l relatively inexpensive standard stockmay beused, i. e. YresistorRl may be 110,000' ohms instead of 100,000-ohms andffso on. Apart from that, resistance and-capacitor values dependen the structure'of the circuitszand their elements and,;therefore, may assume widely varying'values without .exceeding the scope ofthis invention. Y

One is not bound to apply the vdiscrete-potentials'to two electron discharge paths.` Neither is it necessary to-ieed,I the common'foutputoffthese electron discharge paths to a single gas discharge path. If required', any'numbenof'electron discharge paths, including -a 'single onefandI any number of gas-dischargepaths may be provided to cause one or more accurate control operations.

Electron discharge may be substituted for gas discharge orgas discharge for electron discharge; gas discharge operation may also be Areplaced by an electromechanical 'one such was relay'acti'on, Without exceeding the scope ofI this invention.

What we claim is:

1. In a switching system, means iordischarg; ing electrons in two separate streams including grid, cathode'and anode in each stream, a vconomon receiving 'circuit connected to the anfodes, means 'including ymagnet-operated stepping switches for applying a `number of discrete po tentials, one ata timeto each grid, y'means for maintaining the ldiscrete potentials applied "to the grid in one stream in a constant relation with the potentials vapplied tothe cathode in the other stream, the potential maintaining means including a resistance in seriesrwith each grid, a source of D. C. potential connected between said resistance and thel Apotential applying means, and means operated from saidcommon receiving circuitto control at lea-st one of the stepping switch magnets when the discrete potentials appliedfto the different grids bear a predetermined'relationship- 2. A system according to claim 1 in which said D. C. source. of potential- 'includes acapacity shunted balancing resistance, A; C. fed cathode heatingmeans, rectifying means; transformer means having a primary coupled :to the cathode heating means and a secondary coupled through the rectifying means to one end-A of the biasing element, said secondary having a center tap cou-J pled to the other end ofthe biasing element.

3. In a control system, an electron discharge tube containing two like multi-electrode structures, means for applyinguto an input' electrode in each structure-a seriesof discretel potentials, one at a time tov each input electrode, a common receiving circuit including. an; output electrode in each structure'and operatedwhen the potentials applied to 'the inputelectrodesnare substantially equal, a balancing resistance for each input electrode, rectifying means and A. C. fed transformer means connected to apply the same constant biasing potential through said rectifying means and said balancing resistance to each input electrode, said biasing potential being of the order of the potential difference between successive discrete potentials.

4. Apparatus for comparing two potentials to control an automatic switch in accordance therewith, comprising two sources of pluralities of discrete potentials, an automatic switch, electron discharge means for receiving a potential from each source and producing an operating voltage when they are substantially equal, said means comprising electrodes including two cathodes, two control grids and at leas-t one anode which aiord two ldischarge paths, each control grid and each cathodebeing associated with one of the paths, means for heating the cathodes with alternating current, means responsive to the operating Voltage to control the automatic switch including a gas discharge tube having a control electrode, a control gap and a main gap, means for selecting a discrete potential from one of the sources and applying it to the control grid asso- -ciated with one path and to the cathode associated with the other path, means for applying an ungrounded biasing potential between said grid and said cathode, means for applying an ungrounded biasing potential between the other grid and the other cathode, means for operating said automatic switch to successively apply individual potentials from the other source to said other control grid and said other cathode, a common output circuit for the two electron discharge paths including an anode resistance and a source of anode potential connected in series, means for connecting the control electrode of the gas tube to said output circuit at the end of said anode resistance farthest from said source of anode potential, a source of energizing potential for the main gap of the gas tube, the means responsive to the operating voltage also comprising means including a relay connected in series with said main gap for responding to a discharge of the gas tube to arrest the operation of said automatic switch, each of the means for applying biasing potential comprising a transformer including a primary connected to said means for heating and a secondary, at least one condenser and one rectier arranged in series and connected across at lea-st a portion of said secondary, and a biasing resistance connected across the condenser and connected on its one end to a control grid and 10 on its other end to a cathode, whereby when the potentials applied to both grids and both catliodes are substantially equal a gas tube will be ionized to arrest the operation of the automatic switch.

5. Apparatus for comparing as in claim 4, and in which said means for selecting a discrete potential comprises a register switch having a plurality of contacts and a wiper adapted to engage the contacts and means for setting the register switch, one of the sources of discrete potentials is connected to the register switch to apply discrete potentials individually to contacts of said switch, said automatic switch is a selector switch having a plurality of test contacts, and a brush adapted to engage the test contacts, said means vfor operating includes means for moving the brush over the test contacts, the other source of discrete potentials is connected to the selector switch to apply discrete potentials individually to test contacts, the means for responding to a discharge of the gas tube is adapted to disable said means for moving the brush, and there is included a means for preventing false operation of the electron dis-charge means comprising a first circuit including in series a current limiting resistance and a source of mismatching potential and a second circuit including another current limiting resistance, one of said circuits being connected between said wiper and ground and the other of said circuits being connected between said brush and ground.

6. Apparatus for comparing as in claim 4, which also includes means for preventing false ionization of the gas tube if a transient voltage corresponding to said operating voltage should be applied to said control electrode includingv a condenser connected across the control gap and a bleeder resi-stance connected in parallel to said condenser.

7. Apparatus for comparing as in claim 4, in which the electron discharge means comprises a twin diode.

GERALD DEAKIN. HENRY F. HERBIG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,351,016 Deakin June 13, 1944 2,354,682 Herbig Aug. 1, 1944 

